Nowadays, since CO2 emission regulations are being strengthened in response to mounting environmental problems, weight reduction of an automobile body for increasing fuel efficiency is a target to be achieved in the automobile industry. Therefore, there is a growing trend toward using a high-strength steel sheet for automobile parts in order to decrease the thickness of steel sheets, in particular, there is a growing trend toward using a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1180 MPa or more.
A high-strength steel sheet which is used for the structural members and reinforcing members of an automobile is required to have excellent formability. In particular, a high-strength steel sheet which is used for parts having a complex shape is required to be excellent not only in terms of single property such as elongation or stretch flange formability (also referred to as hole expansion formability), but in terms of both elongation and stretch flange formability. Moreover, automobile parts such as structural members and reinforcing members are required to be excellent in terms of impact energy absorbing property. Increasing the yield ratio of a steel sheet, which is a material for automobile parts, is effective for increasing the impact energy absorbing property of the automobile parts. Automobile parts which are manufactured by using a steel sheet with a high yield ratio are capable of efficiently absorbing impact energy with a small amount of deformation. Here, “yield ratio” (YR) refers to the ratio of yield stress (YS) to tensile strength (TS) and is expressed as YR=YS/TS.
Conventionally known examples of a high-strength thin steel sheet having both high strength and satisfactory formability include dual phase steel (DP steel) having a ferrite-martensite structure (Patent Literature 1). The DP steel, which is multi-phase steel having a microstructure including ferrite as a main phase in which martensite is dispersed, has a low yield ratio, high TS, and excellent elongation.
In addition, known examples of a steel sheet having both high strength and excellent ductility include a TRIP steel sheet, which is manufactured by utilizing the transformation induced plasticity of retained austenite (Patent Literature 2). Since this TRIP steel sheet has a steel sheet microstructure including retained austenite, when the TRIP steel sheet is subjected to deformation by performing processing at a temperature equal to or higher than the martensite transformation start temperature, a large elongation is achieved as a result of retained austenite undergoing induced transformation into martensite by stress.